This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The ultimate goal of mobile broadband is ubiquitous and sustainable provision of non-limiting data rates to any person or any terminal at any time. In order to achieve this goal, Ultra-Dense Networks (UDNs) become an important solution after the successful introduction of Long Term Evolution (LTE) for wide-area and local-area access. Through overprovision and related low average loads in an access network, UDNs may create ubiquitous access opportunities which, even under a realistic assumption on user density and traffic, provide users with desired data rates.
Overprovision is achieved by an extremely dense grid of access nodes (ANs). Inter-access-node distances in the order of tens of meters or below may be envisioned, and indoor deployments of one or even multiple access nodes are conceivable in each room. In addition to the increased network capacity, densification—via reduced transmit power—also offers access to vast spectrum holdings in millimeter-wave (mmW) bands and thus the increased data rates.
For example, a several-gigahertz (GHz) spectrum is available in the unlimited 60 GHz band and potentially more in other millimeter-wave bands, enabling multi-Gb/s transmission even with technologies providing moderate spectral efficiency. While schemes with moderate spectral efficiency may be perceived as old-fashioned, they offer robustness and energy efficient data transmission. Furthermore, there are some implementation issues at higher millimeter-wave frequencies that make it very challenging to provide high spectral efficiency. In this sense, the spectral efficiency may be traded for bandwidth.
FIG. 1 illustrates a logical architecture and interface for an example mmW network. As illustrated in this figure, ANs (only two are shown in FIG. 1) are connected with an aggregation node (AGN) via a wireless interface Y1 and the AGN is connected to a network controller (NC) and a local gateway (LGW) via a fixed interface, e.g. extended S1-Mobility Management Entity (MME). The NC is a control node within the mmW network, which is responsible for spectrum selection, routing and resource allocation, etc. The LGW is a user plane node within the mmW network, which has a connection with the external core network.
Since the density of ANs within an mmW network is very high, a user equipment (UE) may have to frequently switch from one AN/AGN to another AN/AGN. The traditional handover procedure defined in LTE is therefore not suitable for mmW networks, which involve relatively complex signaling and take relatively longer time. It would be a great advantage that the whole mmW network looks like an evolved NodeB (eNodeB or eNB) from the external core network point of view, so that UE mobility within the mmW network may be treated using the Radio Resource Management (RRM) protocol. In order to support this requirement, data packet delivery within the mmW network may be done at the radio link control (RLC) layer using the relay Automatic Repeat reQuest (ARQ) mechanism, in which each link hop operates at the physical (PHY)/Media Access Control (MAC) layer, and the end-to-end connection (from AN/AGN to NC or LGW) operates at the Radio Resource Control (RRC) layer for the control plane or at the Packet Data Convergence Protocol (PDCP) layer for the user plane.
The UE mobility procedure within an mmW network is needed when a UE switches from one AN to another AN in the mmW network, especially when the two ANs have no direct connection in between. In this case, the routing path for the UE within the mmW network needs to be changed and the NC must be involved. The traditional handover like procedure is not suitable in the mmW network, as the source AN needs to know the neighborhood relationship with the target AN and needs to establish a forwarding tunnel toward the target AN for context information and data packets. This implies a heavy burden and overhead on each AN.
Instead, the mobility management may be controlled by the NC. The NC is a central control node in an mmW network and thus knows the whole network topology. When a UE is connecting with a source AN, it reports measurement results to the NC. Then the NC can determine if another AN neighboring to the source AN may become a target AN for serving the UE or not. If the UE needs to switch to the target AN, different from the traditional handover procedure, the source AN may not be required to transfer the UE context to the target AN, according to an existing solution proposed in a Patent Cooperation Treaty (PCT) application No. PCT/CN2014/094133. Instead, the target AN obtains the necessary context information from the UE and the NC. The context information to be acquired from the UE may be UE network capability, packet status information, UE historical information, etc. The context information to be acquired from the NC may be QoS related information.
However, for a large-size network, with the centralized control, RRC signaling may have to transmit back and forth over multi-hops with the NC to trigger data plane switching at the LGW during UE switching, this may increase latency in both the control plane and the data plane, consequently causing degraded performance and end user's experience. Besides, in order to have lossless mobility, the LGW needs to retransmit data packets that the UE does not receive from the source AN to the target AN over a new path after the switching procedure, i.e. the transmission starts to be carried out on the new path. This may further increase the latency and degrade system performance.